The present invention relates to an electrophotographic apparatus and a process cartridge for a printer, a copying machine, or a facsimile machine.
In recent years, electrophotographic apparatus, such as a laser beam printer and an LED printer, are widely used. There is a increasing demand for low cost page printers with the spread of personal computers, and a laser beam method has an exposure control with more simple constitution compared with an LED method and can provide inexpensive electrophotographic apparatus. In addition, processing of photograph pictures can now be attained individually due to inexpensive high performance personal computers, and thus a high-resolution printer that can output photograph pictures is also demanded. Since office networks are progressed and two or more users use a printer simultaneously, improvement in its speed is required.
In high-resolution printers, a method of making the spot diameter of an exposure beam small to deal with high resolution has been adopted. When an exposure beam is made to provide a smaller diameter, the diameter of a proper spot is, for example, about 20 xcexcm in 1200 dpi, and about 10 xcexcm in 2400 dpi. In this case, the depth of focus of an exposure beam becomes shallow as the spot diameter becomes small, and hence, in order to form an image on a latent image holding member, a highly precise optical system is needed, which leads to a cost rise. Then, as a method of obtaining a higher pixel density with no cost rise of an optical system, a method is also adopted in which drive current of a light emitting device is weakened so that the output of exposure beams can become small without changing the spot diameter.
Moreover, when the resolution of a device or the process speed is made higher, the problem of a cost rise is brought about. This is because in laser beam printers, an increase in the number of rotation of a scanner needs a larger drive power source for rotation or reinforcement of the rotation support axis.
In order to cope with this problem, a multi-beam method is effective in which scanning is performed with two or more laser beams and static latent images for two or more lines can be simultaneously formed on an electrophotographic photosensitive member.
On the other hand, as an inexpensive charging equipment that charges an electrophotographic photosensitive member, a contact charging method is widely adopted. In this method, a high voltage power source and an ozone filter are not required, and simple constitution, such as a roller type, may be used also in charging members. In this method, uniform charging can be carried out using a method proposed by the inventor of the present invnetion in which voltage applied to contact charging member is oscillating voltage (Japanese Patent Publication No. 3-52058), especially a method in which an oscillating voltage peak value Vpp is two times or more as high as a charging starting voltage Vth when a direct current is applied to the charging member.
However, according to investigation by the inventor of the present invention, vertical streak-like image defects occurred in the second half of repetitive use durability test, when the output of exposure beams is made small to deal with high resolution at an accelerated process speed.
The state of image defects is shown in FIG. 15. Vertical streaks were fine white lines (a) generated at half-tone areas in photograph pictures or the like, and minute scratches (b) were present on the surface of an electrophotographic photosensitive member A corresponding to the streaks.
An explanation of this phenomenon will be given in FIG. 16. FIG. 16 shows an on-off signal of a laser and a static latent image pattern formed on the electrophotographic photosensitive member when a laser beam printer with the resolution of 1200 dpi and the spot diameter of 80 xcexcm was used. The static latent image pattern was formed on the circumference of a section L shown in FIG. 15. The pattern used here for the static latent image pattern was a half tone of one-dot-two-space horizontal lines.
In a portion (c) where electrophotographic photosensitive member got damaged, when a laser beam is irradiated, irregular reflection occurs. Laser beams are reflected by irregular reflection from the electrophotographic photosensitive member surface, and as a result, the amount of light is reduced and insufficient to irradiate a charge-generating layer. Therefore, the potential does not fall to a dark portion potential and development cannot be effected, so that white lines occur.
This phenomenon becomes remarkable as the process speed becomes higher, because the rubbing power with members (for example, charging rollers, cleaning blades, etc.) in contact with the electrophotographic photosensitive member becomes stronger. Moreover, in high durable equipments, the streaks on the electrophotographic photosensitive member surface become gradually deeper and worse in their level.
The present invention was made in order to solve the above-described problems of prior art. An object of the present invention is to provide an electrophotographic apparatus having high resolution, in particular, forming images through scanning with a plurality of beams, which has an inexpensive structure and can output good photograph pictures without image defects such as white streaks even when the process speed is high; an electrophotographic photosensitive member used in the electrophotographic apparatus; and a process cartridge having the electrophotographic photosensitive member.
Another object of the present invention is to provide an electrophotographic apparatus, a process cartridge, and an electrophotographic photosensitive member which can keep the quality of photograph pictures high and have high durability.
Namely, the present invention provides an electrophotographic apparatus comprising an electrophotographic photosensitive member and an exposure means for forming a static latent image on the surface of the electrophotographic photosensitive member by scanning with an exposure beam based on image information, the spot diameter of the exposure beam being 2.5 times or more the size of one pixel of an image formed by the electrophotographic apparatus, wherein
a Fischer hardness of the surface of the electrophotographic photosensitive member is no less than 240 N/mm2,
a NESA sensitivity of the electrophotographic photosensitive member is no less than 2000 Vxc2x7cm2/xcexcJ,
the electrophotographic photosensitive member has at least an electric charge generating layer and an electric charge transportation layer, and
a thickness of the electric charge transportation layer is no more than 30 xcexcm.
Further, the present invention provides a process cartridge which supports as one unit an electrophotographic photosensitive member in which a static latent image is formed on its surface by scanning with an exposure beam from an exposure means based on image information and at least one means selected from a group consisting of a developing means, a cleaning means, and a charging means, and can be mounted on, and detached from, the body of the electrophotographic apparatus, wherein
a spot diameter of the exposure beam is 2.5 times or more the size of one pixel an image formed by the electrophotographic apparatus,
a Fischer hardness of the surface of the electrophotographic photosensitive member is no less than 240 N/mm2,
a NESA sensitivity of the electrophotographic photosensitive member is no less than 2000 Vxc2x7cm2/xcexcJ,
the electrophotographic photosensitive member has at least an electric charge generating layer and an electric charge transportation layer,
a thickness of the electric charge transportation layer is no more than 30 xcexcm.
Moreover, the present invention provides an electrophotographic photosensitive member in which a static latent image is formed on its surface by scanning with an exposure beam from an exposure means based on image information, wherein
a spot diameter of the exposure beam is 2.5 times the size of one pixel,
a Fischer hardness of the surface of the electrophotographic photosensitive member is no less than 240 N/mm2,
a NESA sensitivity of the electrophotographic photosensitive member is no less than 2000 Vxc2x7cm2/xcexcJ,
the electrophotographic photosensitive member has at least an electric charge generating layer and an electric charge transportation layer, and
a thickness of the electric charge transportation layer is no more than 30 xcexcm.